1. Field of Invention
This invention relates to the field of optoelectronic devices and, more particularly, to organic light emitting devices. This invention further relates to methods of forming the organic light emitting devices and displays using such devices.
2. Description of Related Art
Tang and Van Slyke reported efficient electroluminescence from a bilayer organic device in 1987. S. A. Van Slyke et al., xe2x80x9cOrganic light emitting Devices with Improved Stability,xe2x80x9d Appl. Phys. Lett. 69, pp. 2160-2162, 1996. Since that time, organic light emitting devices (OLEDs) have attracted great attention because of their potential toward the fabrication of large-area displays. See, J. R. Sheats et al, xe2x80x9cOrganic Electroluminescent Devices,xe2x80x9d Science 273, pp. 884-888, 1996; J. Salbeck, xe2x80x9cElectroluminescence with Organic Compounds,xe2x80x9d Ber. Bunsenges. Phys. Chem. 100, pp. 1667-1677, 1996; and Z. Shen et al., xe2x80x9cThree-Color, Tunable, Organic Light-Emitting Devices,xe2x80x9d Science 276, pp. 2009-2011, 1997.
To achieve efficient electroluminescence, some known organic light emitting devices include separate layers of a hole transport material (HTM) and an emitting electron transport material (ETM). The structure of such a conventional bilayer organic light emitting device 10 is shown in FIG. 1. The organic light emitting device 10 includes a substrate 12 composed of, for example, glass; an anode 14 on the substrate 12 and typically composed of a transparent conductor, for example, indium tin oxide (ITO); a hole transport material layer 16 on the anode 14; an electron transport material layer 18 on the hole transport material layer 16; and a cathode 20 on the electron transport layer 18 and typically composed of a low work function metal or metal alloy. During operation, an applied electric field causes positive charges (holes) and negative charges (electrons) to be respectively injected from the anode 14 and the cathode 20 to recombine and thereby produce light emission.
In some known organic light emitting devices, the hole transport and electron transport layers are doped with organic dyes in order to enhance the efficiency or to improve the stability of the organic light emitting devices. See the above-described Van Slyke et al. article and also Y. Hamada et al., xe2x80x9cInfluence of the Emission Site on the Running Durability of Organic Electroluminescent Devices,xe2x80x9d Jpn. J. Appl. Phys. 34, pp. L824-L826, 1995, and J. Shi et al., xe2x80x9cDoped Organic Electroluminescent Devices with Improved Stability,xe2x80x9d Appl. Phys. Lett. 70, pp. 1665-1667, 1997.
There have also been attempts to obtain electroluminescence from organic light emitting devices containing mixed layers, i.e., layers in which both the hole transport material and the emitting electron transport material are mixed together in one single layer. See, for example, J. Kido et al., xe2x80x9cOrganic Electroluminescent Devices Based On Molecularly Doped Polymers,xe2x80x9d Appl. Phys. Lett. 61, pp. 761-763, 1992; S. Naka et al., xe2x80x9cOrganic Electroluminescent Devices Using a Mixed Single Layer,xe2x80x9d Jpn. J. Appl. Phys. 33, pp. L1772-L1774, 1994; W. Wen et al., Appl. Phys. Lett. 71, 1302 (1997); and C. Wu et al., xe2x80x9cEfficient Organic Electroluminescent Devices Using Single-Layer Doped Polymer Thin Films with Bipolar Carrier Transport Abilities,xe2x80x9d IEEE Transactions on Electron Devices 44, pp. 1269-1281, 1997. In many such structures, the electron transport material and the emitting material are the same. However, as described in the S. Naka et al. article, these single mixed layer organic light emitting devices are generally less efficient than multi-layer organic light emitting devices.
Moreover, the above-described references have not addressed the stability of these single mixed layer organic light emitting device structures. In fact, studies by the present inventors on organic light emitting devices structures including only a single mixed layer of a hole transport material (composed of NBP, a naphtyl-substituted benzidine derivative) and an emitting electron transport material (composed of Alq3, tris (8-hydroxyquinoline) aluminum) have revealed that these known single mixed layer organic light emitting device structures are inherently unstable. The instability of these devices is believed to be caused by the direct contact between the electron transport material in the mixed layer and the hole injecting contact (comprised of indium tin oxide (ITO)), which results in the formation of the unstable cationic electronic transport material, as well as to the instability of the mixed layer/cathode interface. See, H. Aziz et al., Science 283, 1900 (1999), incorporated herein by reference in its entirety.
There have also been attempts to obtain electroluminescence from organic light emitting devices by introducing hole transport material and emitting electron transport material as dopants in an inert host material, as reported in the above-described article by J. Kido et al. However, such known devices have been found to be generally less efficient than conventional devices including separate layers of hole transport material and emitting electron transport material.
Known organic light emitting devices have relatively short operational lifetimes before their luminance drops to some percentage of its initial value. Although known methods of providing interface layers and doping have increased the operational lifetime of organic light emitting devices to several tens of thousands of hours for room temperature operation, the effectiveness of the known organic light emitting devices deteriorates dramatically for high temperature device operation, as the existing methods used to extend the device lifetimes lose their effectiveness at higher temperatures. In general, device lifetime is reduced by a factor of about two for each 10xc2x0 C. increment in the operational temperature. As a result, the operational lifetime of known organic light emitting devices at a normal display luminance level of about 100 cd/m2 is limited to about a hundred hours or less at temperatures in the range of 60-80xc2x0 C. See the above-described article by J. R. Sheats et al. and also S. Tokito et al., Appl. Phys. Lett. 69, 878 (1996). These operational device lifetimes are unsatisfactory for use of the organic light emitting devices at these high temperatures, where an operational device lifetime in the order of several thousand hours is generally needed for various potential applications of organic light emitting devices.
This invention overcomes the above-described disadvantages of known organic light emitting devices (OLEDs) and provides improved organic light emitting devices having enhanced efficiency and operational lifetimes. In addition, the organic light emitting devices according to this invention can provide operational stability at high temperature device operation conditions.
Accordingly, the organic light emitting devices according to this invention can be used for various applications, and especially high-temperature technological applications that require high-temperature stability over significant lifetimes.
Exemplary embodiments of an organic light emitting device according to this invention comprise a mixed region comprising a mixture of a hole transport material and an electron transport material; one of the hole transport material and the electron transport material of the mixed region is an emitter. The organic light emitting device also comprises at least one of a hole transport material region and an electron transport material formed on the mixed region. In embodiments, an anode contacts either the hole transport material region or a surface of the mixed region, and a cathode contacts either the electron transport material region or another surface of the mixed region.
The organic light emitting devices according to the invention can be utilized in various devices such as displays that typically are operated over a broad range of temperature conditions. The operational stability at high temperature conditions provided by the organic light emitting devices of this invention enables embodiments to be used at high temperature applications for extended periods of time.
This invention also provides methods of forming the organic light emitting devices. One exemplary method according to exemplary embodiments of this invention comprises forming a mixed region comprising a mixture of a hole transport material and an electron transport material, one of which is an emitter. A hole transport material region and/or an electron transport material region is formed on the mixed region. An anode is formed in contact with either the hole transport material region or with a surface of the mixed region. A cathode is formed in contact with either the electron transport material region or with another surface of the mixed region.